The present invention relates to apparatus and methods for adjusting filters acting as equalisers or predetection signal processors in receivers for data transmission links such as modems for operation for example over telephone, optical fibre or high frequency radio systems.
When a data transmission system operates by sending a serial stream of signal elements in the form of binary or multi-level pulses, four types of distortion occur: attenuation distortion, phase distortion, noise multiplication and noise addition. A solution which has been used to overcome attenuation and phase distortion uses a linear equaliser whose attenuation versus frequency and phase versus frequency characteristics are the inverse and negative, respectively, of those of the data link. The use of an equaliser with an attenuation characteristic of this type leads to the enhancement of noise at frequencies where relatively high attenuation is introduced by the data link.
Two types of equaliser-detector combinations are commonly used: firstly a linear feed-forward transversal filter followed by a maximum likelihood detector such as a detector using the Viterbi algorithm or else a near maximum likelihood detector such as a reduced state Viterbi algorithm detector; and secondly a non-linear (decision-feedback) equaliser and a simple detector, the non-linear equaliser employing both a feed-forward transversal filter and a feed-back transversal filter. Where either the maximum likelihood detector or equaliser are used the linear feed-forward transversal filter ideally attempts to correct only the phase distortion caused by the data transmission link. The near maximum likelihood detector operates to provide optimum detection, whereas the feed-back filter in the equaliser attempts to provide accurate equalisation of the data transmission link and feed-forward filter in cascade.
The maximum likelihood detector suffers from the disadvantage that it requires too much electrical storage and a large number of operations per symbol transmitted. The non-linear equaliser is sub-optimum, essentially because a received signal element is here detected solely from its first component (at the output of the feed-forward filter) the remaining components being ignored in the detection process.
It has become clear that for most reliable operation of a maximum or near maximum likelihood detector or else of a non-linear equaliser, the feed-forward transversal filter must be adjusted to act as an all pass network which gives a resultant minimum phase response (that is no attenuation occurs at any frequency transmitted).
A gradient method is known for adaptively adjusting the feed-forward filter and providing a sampled impulse-response of the data transmission link and the linear feed-forward transversal filter required for use either by the near maximum likelihood detector or by the feedback transversal filter. This method is satisfactory where the data link has a high signal to noise ratio but unfortunately over poorer telephone circuits where it is most important to achieve the best available tolerance to noise, the degradation in tolerance becomes significant. Furthermore an unduly long training signal is required at the start of transmission for the initial adjustment of the feed-forward transversal filter in an adaptive system. The duration of the training signal can be substantially reduced by using a Kalman filter but only by considerable increase in equipment complexity.
The subjects of distortion in data transmission systems, non-linear equalisers and maximum likelihood detectors are discussed in the book "Advanced Data-Transmission Systems", by A. P. Clark, published by Pentech Press in 1977. The z transform, an essential technique in the present invention, is also discussed.